1. Field of the Invention
This invention relates to an inductor based switching regulator in which the output voltage can be less than, equal to, or greater than the input supply voltage. In particular, this invention relates to a buck-boost (step down-step up) type switching regulator which exhibits better performance than those of the prior art.
2. Background of the Related Art
Most prior art buck-boost regulators utilize voltage mode control where the inductor current is not measured at all or at least not utilized in the control process. A typical configuration of a conventional buck-boost switching regulator is shown in FIG. 1, which shows a functional block diagram of TPS6300X of Texas Instruments. The regulator utilizes MOS transistors for switching and the current flowing in the input MOS transistor switch. The gate controller directly uses VOUT to control each of the MOS transistors. It is, however, noted from FIG. 1 that this regulator does not utilize an average current flowing in an inductor. Some other prior art switching regulators utilize instantaneous (often referred to as peak) current control where the switch or inductor current is measured and its time domain waveform is used directly in the PWM modulator as the “clock” waveform.
Voltage mode control has a slow transient response, which changes greatly between continuous current mode (CCM) and discontinuous current mode (DCM) operation unless the loop compensation is also changed. With voltage mode control, the pulse width modulation (PWM) gain at the PWM modulator varies with the supply voltage, thereby requiring supply voltage feed-forward operation in the controller for reasonably good performance. However, current measurement or slope compensation is not required for operation of a voltage mode controller.
Conventional “peak” current mode operation has inherently good transient response. Such devices can operate in both continuous and discontinuous inductor current mode without compensation change. This mode of operation also utilizes the measured current waveform for the “clock” function in the PWM modulator, and thus has inherent supply voltage magnitude compensation and fast transient response. However, the conventional “peak” current mode requires wide bandwidth current measurement, is very sensitive to noise on the current waveform, requires slope compensation in the PWM modulator to avoid instability when operating over a wide range of switch duty cycles and is difficult to configure for buck-boost operation.
On the other hand, average current mode is advantageous particularly for buck-boost functionality since it uses lower bandwidth, time averaged measured inductor current in the controller, operates in both CCM and DCM without compensation changes, requires no slope compensation regardless of duty cycle and has a quite good transient response. However, conventional average current mode devices suffer from various implementation challenges. Specifically, such devices require continuous inductor current measurement, not the intermittent switch currents; require supply voltage feed forward to maintain constant PWM gain at varying supply voltage; and need a more complex PWM modulator, especially for buck-boost operation.
Further, both CCM and DCM operation are required at fixed clock frequency for good current efficiency at small load and fast programming of the output voltage. DCM offers better light load efficiency because it reduces I2R losses in the switches and inductor and also reduces output voltage ripple magnitude. However, it can not easily provide negative polarity inductor current when needed to program a rapid decrease in the output voltage value. CCM readily provides for rapid increases and decreases in the output voltage in response to both programming and load changes. In CCM, the amplitude of the inductor ripple current, which is approximately constant, is a relatively small percentage of larger load currents. If mode transition between DCM and CCM is automated, CCM can be used automatically during transients without significantly reducing average power efficiency.
Therefore, there is a need for a buck-boost switching regulator utilizing average current mode control without excessive circuit complexity and which provides for operation in both CCM and DCM.